Carbon monoxide (CO) is one of the most common impurities in hydrogen (H2) streams that have a great negative impact on the performance and durability of polymer electrolyte membranes (PEM). Just 10 ppm of CO in H2 fuel stream can cause degradation in the performance and durability of fuel cells. The on-board removal of low level CO from H2 rich stream to less than 10 ppm is a challenging task. The current available technologies are precluded from applicability because of their high cost, poor selectivity, and elevated temperature and pressure requirements. In addition, these technologies suffer from significant H2 loss.
H2 is generally generated from hydrocarbons, natural gas, or methanol via steam methane (CH4) reformation (SMR) (H2O+CH4═CO+3H2) followed by a water gas shift (WGS) reaction (H2O+CO═CO2+H2). The gas effluent contains approximately 2.0 vol. % CO in excess H2. This low concentration of CO in the H2 outlet stream from WGS can not be avoided and a deep removal process is required. Apparatus, system and processes for eliminating the CO in the H2 fuel stream are beneficial in increasing the performance and durability of polymer electrolyte membrane fuel cells.
Present technologies for on board CO removal can be separated into off-fuel cell approach and on-fuel cell approach. The former removes CO from the H2 fuel in advance of the fuel cell apparatus, while the latter process is integrated within the fuel cell. In the off-fuel cell CO removal, the goal is to maximize the adsorption of CO on the catalyst surface. However, in the on-fuel cell CO removal, the objective is to minimize the CO adsorption on the fuel cell anode-catalyst.
The prior art technologies for the on-board removal of CO for automotive applications have limitations. Considering a 100 horsepower (75 kW) automobile powered by H2, under the assumption that an H2 fuel cell is operated at 0.75 V with a 50% overall efficiency, the electrical current required from the fuel cell can be calculated as 200 kA. Based on Faraday's Law, theoretical (or minimum) H2 flow rate required for a PEM fuel cell to generate 1 A current can be calculated for both anode and cathode reactions as:
Based on an anode hydrogen oxidation reaction: H2=2H++2e−, one mole of H2 makes 2 equivalents (n=2 eq./mole). Therefore, a 1 A (1 A=1 coulomb/sec) current is produced by an H2 flow (at standard conditions):1 Coulomb/sec×60 sec/min×22,414 ml/mole/(96,458/eq.×2 eq./mole)=6.97 mL/min.200 kA requires an H2 flow of: 200×1000×6.97 mL/min/(1000 mL/L)=1394 L/min.
If the calculation is based on a cathode reaction: 4H++O2+4e−=2H2O, n=4 eq./mole, then 1 A current requires:1 Coulomb/sec×60 sec/min×22,414 ml/mole/(96,458/eq.×4 eq./mole)=6.97/2 mL/min.
So 200 kA current requires: 6.97/2×200/2=1394/2 L/min.
It should be pointed out that the above flow rate (1394/2 L/min) represents minimum O2 flow that is equal to 2 times of H2 based on the reaction: 2H2+O2=2H2O. So the theoretical H2 flow rate should be:2×O2 flow rate=2×(1394/2)=1394L/min.
Assuming CO removal residence time is 10 seconds; the reactor volume can be calculated as 232.4 L. If the reaction rate could be reduced to 0.1 second, the reactor volume could be reduced to 2.3 L.
In order to have a better understanding of the challenges and the principles for the removal of low concentration CO in an H2 stream, some analyses and discussions are needed.
Off-fuel cell CO removal methods include Pd-based membrane purification, water gas shift (WGS) reaction, catalytic methanation, and catalytic preferential CO oxidation as follows:CO+H2O═CO2+H2(Water Gas Shift Reaction)  (1)CO+3H2═CH4+H2O(CO methanation)  (2)CO+O2═CO2(Preferential Oxidation)  (3)
Pd-based membrane purification method is expensive and requires both high operating temperature and a high-pressure differential. Therefore, it is not suitable for on board applications. Thermocatalytic WGS processes, including high temperature and low temperature WGS reactions, are suitable for treatment of different CO concentrations. The low temperature WGS is normally operated at temperatures as high as 200° C. to ensure a reasonable reaction rate, and therefore can not be used for on-board applications based on the temperature requirement. Catalytic CO methanation is the hydrogenation of CO on supported metal catalysts in H2 fuel. The advantages of the methanation process are that it avoids the introduction of O2 or air to the fuel cell system, and the generated methane (CH4) gas does not deactivate the fuel cell anode catalyst. However, its disadvantages are the consumption of H2 and the requirement of high temperature. The CO preferential oxidation process uses less than 2 percent of air by volume mixed with the H2 fuel stream and fed into a metal-based catalyst. This catalyst preferentially adsorbs CO, which then reacts with O2 to form CO2. The typical metal catalysts for the oxidation of CO are alumina-supported Pt-group metal catalysts and metal oxide-supported gold (Au) catalysts. In this oxidation process, part of H2 is oxidized to produce water resulting in a fuel loss. A great effort has been devoted to reduce the temperature to lower than 80° C. for the preferential process in order to be applicable in an H2 PEM fuel cell system. Some important advances of prior art catalytic preferential CO oxidation are summarized below.
Fenton et al. reported an approximately 100% CO conversion with an Ir/COOx-Al2O3/carbon catalyst at an O2/CO ratio of 1.5 in a humidified H2 environment and a temperature near 75° C. as described in C. He, H. R. Kunz, and J. M. Fenton, Selective Oxidation of CO in Hydrogen Under Fuel Cell Operating Conditions, J. Electrochem. Soc., 148(10) (2001), pp. A1116-A1124. They also showed that Co—Ru/C catalysts are very effective for CO methanation. Muradov and co-workers investigated the catalytic activity of a wide range of carbon-based materials and examined their structural and surface properties as described in N. Muradov, F. Smith, and A. T-Raissi, Catalytic Activity of Carbons for Methane Decomposition Reaction, Catal. Today, 102-103 (2006), pp. 225-233. Chen and co-workers reported a 100% conversion of CO oxidation using 7% CuO/CeO2 catalysts in an H2 rich environment (H2/CO/O2/He=50/1/1/48) at 87-147° C. as described in Y. Chen, B. Liaw, and H. Chen, Selective Oxidation of Co in Excess Hydrogen Over CuO/CexZr1-xO2 Catalysts, Int. J. Hydrogen Energy, 31 (2006), pp. 427-435. Furthermore, the partial substitution of the Ce lattice with Zr+4(7% CuO/Ce0.9Zr0.1O2) resulted in 100% CO conversion at approximately 77° C. Zhou et al. showed that CO conversion in excess H2 can reach up to approximately 99.5% at a temperature range between 130-150° C. in the presence of Co—Ni supported activated carbon (AC) catalysts as described in G. Zhou, Y. Jiang, H. Xie, and F. Qiu, Non-noble Metal Catalyst for the Carbon Monoxide Selective Oxidation in Excess Hydrogen, Chem. Eng. J., 109 (2005) pp. 141-145. Goerke reported a 95% selective oxidation of CO in micro-channeled reactors using Ru/ZrO2 catalysts at 150° C. and average residence time of 14 ms as described in O. Goerke, P. Pfeifer, and K. Schubert, Water Gas Shift Reaction and Selective Oxidation of CO in Micro reactors, Appl. Catal. A: General, 263 (2004) pp. 11-18.
However, it is should be noted that it is difficult for any of these three methods to completely remove CO because, fundamentally, the ppm level of CO is thermodynamically stable in an H2 stream at ambient conditions. To remove low concentration CO from an H2 fuel stream requires a two-step process in order to overcome these thermodynamic obstacles. The first step is the preferential adsorption of CO on metal-based catalysts to increase the CO concentration locally because CO has a higher catalytic adsorption capability than H2. The second step is the thermochemical conversion of CO to CO2 (Reaction 2) or CH4 (Reaction 3). The two-step process can be described in Reactions 4 and 5 or Reactions 4 and 6.H2+CO(ppm)+M(catalyst)=H2+M-CO(preferential adsorption)  (4)M-CO+O2═CO2+M(CO selective oxidation)  (5)M-CO+3H2═CH4+H2O+M(CO methanation)  (6)
Note that Reaction 4 is favored at low temperatures to ensure the adsorption rates, whereas CO oxidation and methanation require a higher temperature to enhance reaction kinetics. Therefore, there exists a contradictory condition favoring both CO adsorption and the CO reaction kinetics. On the other hand, it is essential to recognize that for the on-board removal of CO the reaction temperatures can not exceed the fuel cell optimal operating temperature of 80° C. If the reaction temperature is at 80° C. or below, the low reaction rates for Reactions 5 or 6 would require large reactor volume or complicated reactor configurations to compensate the slow reaction rate.
Low-level CO in an H2 stream can also be removed on fuel cell anodes. The advantage of on-fuel cell CO treatment is that no additional processing is required. Three fundamental technologies have been reported for on-fuel cell applications: high temperature process, air- or O2 bleeding, and anode catalyst alloying.
In the high temperature process Fenton and co-workers showed that CO adsorption on fuel cell anode catalysts was reduced at temperatures higher than 100° C., thereby alleviating the CO poisoning effects. This is due to the fact that CO adsorption on a Pt catalyst exhibits high negative standard entropy. However, increasing the PEM fuel cell operating temperature might have some adverse impacts on fuel cell performance. Firstly, higher operating temperature greatly increases the resistance of the Nafion® membrane, resulting in a reduction of fuel cell performance. In order to maintain the membrane's low resistance a 100% relative humidity is preferred. When temperature is above 100° C., maintaining high humidity for a PEM fuel cell requires a system pressure greater than 1 atm, which again reduces the efficiency of the fuel cell. Secondly, operating a PEM fuel cell at temperature greater than 100° C. will enhance the aggregation rate of Pt particles as well as the Pt dissolution in the fuel cell electrocatalyst layer, both of which decrease the performance of the cell. Finally, above 100° C. PEM fuel cells suffer a higher rate of membrane degradation, shortening their long-term stability.
In the approach of air or O2 bleeding, air or O2 is introduced into the H2 stream feed to the anode of a PEM fuel cell to oxidize CO adsorbed on the anode catalyst. This technology has been extensively reported and the results have shown some alleviation of the deleterious effect of CO in the H2 stream. However, since the H2 combustion limit is only 5% of O2 in the H2 stream, a malfunction of the O2 inlet flow could result in very undesirable consequences. Also, as indicated in literature, air-bleeding technology is only effective at a very low CO level (i.e. less than 50 ppm) and at low H2 flow rates. As discussed previously, a hydrogen-powered vehicle requires a very high H2 flow rate (minimum 1394 L/min for a 75 kW vehicle). Therefore, air-bleeding technology is unlikely to be suitable for on-board CO removal in a PEM fuel cell system.
For the anode catalyst alloying approach, considerable efforts have been made to develop CO tolerant electrocatalysts. It has been found that adding Ru, Rh or Ir catalysts to the Pt anode reduces CO poisoning, but it can not fundamentally eliminate CO poisoning. Other alloys such as Pt—Sn and Pt—Mo have been investigated. Still, the Pt—Ru alloys are the most promising candidates and have attracted the most attention. Furthermore, at an 80° C. fuel cell operating temperature the Pt alloy method is unable to completely resolve the CO poisoning issue.
What is needed is a process for removing low level CO to improve the performance of PEM fuel cell to the level of a pure H2 stream.